The present invention relates to extrusion screws. Generally in extruders, plastic material in pellet form is fed from a hopper into a barrel or cylinder where the material is progressively heated so that it will melt and can be fed by an extrusion screw into a die to form different types of shapes. The pellets are heated by the barrel and by some frictional heating.
Where attempts have been made to provide the plastic material in powder form, it has been considered necessary to provide vents in the cylinder. The flights on the screw in the vent zone of the cylinder run partially filled and at zero pressure. Vents are provided in the vent zone to permit gases to escape from the melted plastic. These gas pockets escape the melt and travel spirally around the partially filled channel until they can escape out of the vent hole in the cylinder.
It would be advantageous from a cost savings and material usage standpoint to have a cylinder without vents and to have an extruder wherein the plastic material is in powder form.